This invention relates, in general, to power plants which use a deaerator for heating power plant feedwater and for removing oxygen from the feedwater and, in particular, to a control for reducing condensate flow to the deaerator during transient conditions such as reductions in load.
In a power plant, the deaerator is a feedwater conditioning device which causes the removal of oxygen from turbine condensate and provides direct contact feedwater heating. The deaerator is in place between the turbine condenser and the power plant boiler and, hence, receives condensate and outputs feedwater. The deaerator may be a two chamber pressure vessel comprising a deaerating section and a storage tank. The deaerating section and storage tank are interconnected by pressure equalizers and a drain. Normally, the deaerating section is supplied with steam taken from a flash tank or a turbine extraction port.
If there is a reduction in turbine load, there will also be a reduction in available supply steam to the deaerator. This condition in steam pressure is immediately transmitted through the equilizers to the storage tank which is in a saturated condition. The liquid in the storage tank begins flashing steam which then rises into the equalizers. If the pressure drop across the equalizers exceeds the static head in the deaerating section, the incoming condensate will be "backed up" and cause flooding in the deaerating "spray tray" section. This condition has resulted in the dislodging of spray trays.
With a loss of load on the turbine, any feedwater heaters upstream from the deaerator will also lose their source of heat from the turbine; i.e., extraction steam. This will then result in a condensate temperature decrease which will further aggravate the pressure decay-flashing syndrome. The temperature deficiency in the incoming feedwater may be three times the normal design temperature difference. This means that roughly three times the steam flow is needed to compensate for the loss of feedwater heating. Pressure drop across the trays is proportional to the square of the flow which then means that the increase in pressure drop across the trays is approximatey nine times the normal amount.
The foregoing problems are further aggravated as the storage tank water level falls due to the inhibited drain of condensate into the storage tank while the boiler feedpump demands remain the same. The water level controller will see this falling water level and try to correct the situation by increasing condensate in-flow, thereby putting further energy demands on the system. From the foregoing, it can be seen that single-element control; i.e., level control, is inadequate except for steady state conditions. Conventional three-element controls which measure in-flow, out-flow and level are also inadequate under the described circumstances.